This invention relates generally to power supplies and voltage converters, and, in a specific implementation, to circuits that drive electroluminescent lamps with a high alternating polarity voltage from a low voltage battery supply.
A type of electroluminescent lamp that is widely used in numerous applications is a thin structure formed of a layer of electroluminescent material sandwiched between two conductive electrode layers. One of the electrodes is optically transparent to the resulting luminescence. When a sufficiently high voltage is applied across the two electrodes, a resulting electric field causes the intermediate electroluminescent layer to emit light that is visible through the transparent electrode. The brightness of the emission depends upon the magnitude of the voltage applied. Very little current is consumed in the process. The polarity of the supply voltage is alternated at a sufficiently high rate to prevent a build up of charge that causes the intensity of the luminescence to significantly diminish. A power supply especially designed for driving electroluminescent lamps is generally used.
Applications of this type of lamp include the back lighting of portable electronic devices such as watches, telephones, pagers, and the like. Such lamps are also used in automobile dashboards to provide back lighting of various displays. Included among other applications are displays where the lamps are either shaped or masked to form some symbol, letter or number. In most applications, the power supply to the lamp is required to convert a low direct current battery voltage, such as 1.5, 3.0 or 12 volts, into a voltage across the electrodes of the lamp that is 100 volts or more, and having an alternating polarity. A frequency in a range of about 100-1000 Hz. is generally used. Too high a frequency diminishes the life of the lamp and too low a frequency causes visible flicker.
A typical electroluminescent lamp driver uses the fact that this type of lamp has the electrical characteristics of a capacitor. An inductor is connected to a low voltage battery through a transistor switch. This switch is repetitively turned on, to store energy from the battery in the inductor, and then quickly and sharply turned off to generate a high voltage spike or pulse. A series of such pulses applied to the lamp causes a charge to build up in its equivalent capacitor and thus increase the voltage across its electrodes. The high voltage causes the lamp's electroluminescent material dielectric to emit light. The polarity of the charge accumulation in the lamp capacitor is periodically reversed by alternating the polarity of the voltage applied to the lamp, through use of a bridge switching circuit or the like.
It is a primary object of the present invention to provide such a power supply for electroluminescent lamps that regulates and controls the maximum voltage that results from the accumulated charge.
It is another object of the present invention to provide such a voltage controlled power supply that itself consumes little power, thus extending battery life.
It is a further object of the present invention to provide such a power supply on a single integrated circuit chip with only the battery and inductor outside of the circuit chip.
It is a more general object of the present invention to provide an integrated circuit voltage converting techniques of a general application.